In the field of electrophotographic color printing using liquid color toners, two different technologies have been developed for transferring a developed color image from the surface of a photoconductive drum or belt to an adjacent print media. One of these two technologies uses a direct transfer method wherein the unprinted media is passed directly between the surface of a photoconductive drum or belt and a transfer roller where a color image is directly transferred from the surface of the photoconductive drum or belt to the media. One example of such a direct transfer process is disclosed and claimed in the Camis U.S. Pat. No. 5,115,277 issued May 19, 1992, and entitled "Electrostatically Assisted Transfer Roller and Method for Directly Transferring Liquid Toner to a Print Medium", assigned to the present assignee and incorporated herein by reference.
The other of these two technologies uses an intermediate transfer member (ITM) which is positioned between the surface of a photoconductive drum and a transfer roller and is operative firstly to receive the developed color image from the surface of the photoconductive drum and secondly to transfer the developed color image from its own surface to the media which passes between the surface of the intermediate transfer member and an adjacent transfer roller.
One disadvantage of using either one of the above two image transfer methods is that the photoconductive drum material must be insensitive to a carrier fluid, such as an Isopar or equivalent carrier fluid as well as sub-micron toner particles. In addition, in the direct transfer case, the photoconductive drum should also possess a surface preferably having more or less ideal release properties in order to eliminate toner adhesion problems. Other important factors when using a direct transfer process are that direct transfer methods require some means for heating the media, and this requirement in turn can cause both physical and electrical damage to the photoconductive drum material.
It should also be mentioned that in either the above direct transfer or indirect transfer cases, the toner charge level is an extremely important parameter inasmuch as it has a direct critical effect upon toner mass transfer. Using the intermediate transfer method, this process is also not immune to heat damage problems, and a high level of toner charge control is required since this charge control also controls the electrostatic transfer of the liquid color toners from the surface of the photoconductive drum to the intermediate transfer member. However, the direct transfer method of image processing is even more dependent upon the toner charge level since it requires very specific levels of voltage on the photoconductor in order to achieve optimal toner-on-toner multi-layer development. These levels of toner charge are also critical to the direct transfer of color images to the media. Also, the use of these direct transfer techniques has an additional problem in that the media comes into direct contact with the photoconductor, and this can cause undesirable wear on the photoconductor.